A lighthouse at sea doesn't just rotate: its lamp itself dims and brightens on its own. Magnetic white dwarfs, the ultra-dense remnants of stars like our Sun, behave in much the same way. Their powerful magnetic fields can convert light into axions — hypothetical particles that could explain the Universe's hidden mass. But how can we tell what caused the change in brightness: the exotic conversion of photons or the ordinary "trembling" of the star itself?
A team of researchers studied the star PG 1015+014 using data from the TESS space telescope. They built a mathematical model that simultaneously accounts for both the star's rotation and its own complex flickering — not just a smooth sine wave, but a curve with additional harmonics (extra waves, like overtones of a string). It turned out that if you simplistically assume the star's background is simple, you can mistakenly see a hint of axions. But as soon as they added the second harmonic — the second "overtone"—to the model, the spurious signal vanished.
This approach yielded some of the strictest constraints on axions with masses below a micro-electronvolt (that's billions of times lighter than an electron). The scientists also derived a simple formula to predict in advance which of the many magnetic white dwarfs observed by TESS are best suited for future hunts for these ghostly particles.
The magnetic field of a white dwarf can be a billion times stronger than Earth's — powerful enough to make a beam of light momentarily become an axion and back.